Open source helps, but if you didn't build it yourself, you'll need to trust whoever did. F-Droid reproducible builds help in that you only need to trust either F-Droid or the developer, not both.
The browser tends to be safer because it has a stronger sandbox than native apps on a mobile OS. It's meant to be able to run potentially malicious code with a very limited blast radius.
Open source helps, but if you didn't build it yourself, you'll need to trust whoever did. F-Droid reproducible builds help in that you only need to trust either F-Droid or the developer, not both.
The browser tends to be safer because it has a stronger sandbox than native apps on a mobile OS. It's meant to be able to run potentially malicious code with a very limited blast radius.